(1) Field of the Invention
The present invention relates to a system for controlling pitch variation of the blades of a propeller, to a propeller, and to an aircraft.
The invention is thus situated in the narrow technical field of controls for a propeller.
It is conventional to use propellers that are arranged on a lift surface of an aircraft in order to propel it. By driving the movement of air through itself, each propeller delivers a force suitable for generating traction or propulsion, and consequently for causing the aircraft to move forwards in translation.
Thus, a propeller generally comprises a hub secured to a plurality of blades, the hub being covered by a conical fairing.
(2) Description of Related Art
In the early days of aviation, propellers were provided with a plurality of blades at a pitch that was fixed, the blades and the hub forming a single block. Such propellers of a first type were thus known as “fixed-pitch propellers”.
The pitch of the blades was consequently “frozen” at a value determined when the propeller was manufactured. The airfoils of such a propeller can be provided with a variety of “settings”. Depending on the intended mission, a pilot can choose to install a propeller of small pitch to enhance climbing, or a propeller of large pitch in order to optimize cruising flight.
In addition to not being able to have a propeller that is optimized for all missions, it can readily be understood that that first type of propeller presents difficulties, since the entire propeller, and in particular the block comprising the blades and the hub, needs to be changed in order to go from one configuration to another.
A significant improvement to that first type of propeller is known. It comprises a second type of propeller known as a “ground adjustable pitch” propeller. The pitch of the blades of such a propeller can thus be adjusted on the ground. By slackening a collar clamping the blades, it is possible to pivot the blades into a desired position in order to change their pitch.
Compared with the first type of propeller, that second type avoids the need to remove the propeller. Nevertheless, that adjustment clearly cannot be performed in flight.
A third type of propeller known as a “variable pitch” propeller was then devised. The aircraft has a system for controlling pitch variation that enables the pitch of the propeller blades to be varied while in flight.
Conventionally, such a pitch variation control system has a hydraulic pump activated by the pilot via a lever, a hydraulic chamber provided under the nose cone of the propeller, and a piston connected to the blades via connecting rods.
Depending on the order given by the pilot, the pump injects fluid into the hydraulic chamber via a flexible pipe or hose. The resulting variation in the pressure in said hydraulic fluid causes the piston to move. The blades are then caused to pivot about their respective pitch variation axes by the piston.
That type of propeller thus enables the pitch of the blades to be varied in flight so as to go from a small pitch on takeoff to a large pitch in cruising flight.
Furthermore, it should be observed that in the event of the power plant driving the rotation of the propeller, the blades can be set at a pitch for providing minimum resistance to the relative wind, thereby minimizing drag. This configuration is referred to as “feathering” and the various airfoils of the blades of the propeller are held in “the eye of the wind”.
Nevertheless, that third type of propeller is not entirely satisfactory. If during cruising flight the pilot raises the nose of the aircraft, the speed of rotation of the propeller drops and the aircraft loses speed.
Consequently, a fourth type of propeller has been devised for maintaining optimum propulsion or optimum traction as a function of the orientation of the propeller, where the fourth type of propeller is referred to as a “constant-speed” propeller.
As with the third type, a hydraulic device is provided for varying the pitch of the blades of the propeller in flight.
Furthermore, the pilot controls a throttle lever in order to adjust the power delivered by the power plant of the aircraft.
Regulator means are then implemented to control both the power of the power plant and the pitch of the blades in order to maintain the speed of rotation of the propeller constant.
Optionally, the aircraft includes a lever that the pilot can use for setting the speed of rotation of the propeller.
The pitch variation control system used in the third and fourth types of propeller is effective. Nevertheless, it can be difficult and sometimes even impossible to implement.
If the hydraulic fluid feed needs to pass via the power transmission shaft driving rotation of the propeller, then arranging a hose can be seen to be impossible since the hose would need to perform rotary motion.
Furthermore, for reasons of safety, it can be necessary to duplicate pitch variation systems, which is found to be difficult.
Consequently, the pump and hose assembly has been replaced by a hydraulic directional control valve with slides and a link tube including channels for feeding a hydraulic chamber arranged in the hub of the propeller.
The link tube thus has a first end connected to a control piston defining said hydraulic chamber and a second end that rotates and that moves in translation inside the valve.
Document FR 2 927 879 describes a rigid hydraulic control shaft having internal grooves for forming channels.
The hydraulic valve then includes a control rod operable by a pilot via a flight control. As a function of its position, the control rod may connect the second end of the link tube to a fluid feed circuit in order to increase the pressure that exists in the hydraulic chamber, or else it may connect the second end of the link tube to a fluid return circuit in order to reduce the pressure that exists inside the hydraulic chamber.
For example, by pushing the control rod, a pilot connects the hydraulic chamber with the feed circuit. The channels in the link tube thus convey fluid under pressure into the hydraulic chamber of the piston so as to increase the pitch of the blades towards a large pitch.
Conversely, by pulling on the control rod, a pilot connects the hydraulic chamber to the fluid return circuit. Under the effect of a return spring and of aerodynamic forces, fluid is expelled from the hydraulic chamber. The piston reverses together with the control plate, thereby reducing the pitch of the blades towards a small pitch.
It should be observed that the link tube moves together with the piston. As a result, the channels conveying fluid to the propeller hub perform both rotary movement and movement in translation relative to the control rod. It is therefore difficult to achieve good sealing.
Furthermore, it can be understood that it does not suffice for the pitch-changing system to be capable of changing the pitch of the blades, it must also be capable of moving the blades at a speed that is appropriate for the maneuverability requirements of the aircraft.
Furthermore, the pitch-varying system is difficult to mount. In order to mount the hydraulic valve, it is necessary firstly to mount the power block including the piston inside the hub, and then the link tube, and finally the hydraulic valve. Consequently, the hydraulic valve is mounted piece by piece.
Sealing between the various members of the system, and sealing between the members of the system and the outside of the system cannot be verified until that moment. It would appear to be impossible to test the power block or the valve for sealing in the absence of the link tube. If any one of the parameters is unsatisfactory, it is then necessary to disassemble the system in full in order to verify it.
Furthermore, the combined movements in rotation and in translation of the link tube inside the control rod can, over time and as a result of wear, give rise to an increase in control forces and might even lead to jamming of the control rod on the link tube.